1. Field of the Invention
The invention relates to a method and apparatus for locating a fracture site in a human or animal bone and for measuring the increase in mechanical strength at the fracture site during the healing process. More particularly, the invention uses an electromagnetic field to non-invasively measure impedance changes at a bone fracture site during the healing process.
2. Description of the Prior Art
Currently the prior art does not teach an effective method or apparatus for measuring non-invasively the mechanical strength of a bone fracture during the healing process. X-ray techniques are of some assistance but fail to quantitatively indicate mechanical strength. In most cases, a physician maintains the broken bone in a cast for what is conservatively a sufficiently long time for mechanical strength to return to the fracture site. However, such a technique is inadequate for certain patients, including the elderly, who have a healing process which proceeds at an unknown rate which is however slower than normal. Similarly, it would be advantageous to have an early removal of a cast so that the patient could return to normal activities. This would be particularly true of athletes who could return to normal professional activities as soon as adequate mechanical strength had returned to the fracture site.
As will be discussed in detail subsequently in this application, Applicants have related the impedance change at a fracture site with changes in mechanical strength during the healing process. To non-invasively detect such an impedance change, Applicants have invented a method and apparatus which uses an electromagnetic field for sensing such an impedance change at the fracture site. U.S. Pat. No. 3,735,245 entitled "Method and Apparatus for Measuring Fat Content in Animal Tissue Either in Vivo or in Slaughtered and Prepared Form" invented by Wesley H. Harker, teaches that the fat content in meat can be determined by measuring the impedance difference between fat and meat tissue. The Harker apparatus determines gross impedance change and does not provide adequate spatial resolution for the present use. U.S. Pat. No. 4,240,445 teaches the use of an electromagnetic field responsive to the dielectric impedance of water to detect the presence of water in a patient's lungs. However, such an apparatus does not detect the conductivity variations required in the present invention. U.S. Pat. No. 3,789,834 relates to the measurement of body impedance by using a transmitter and receiver and computing transmitted wave impedance from the electrical or magnetic field generated. However, the antenna pickup would receive extraneous noise rendering it inappropriate for the present use. None of the above cited references contemplate measuring the mechanical strength of a bone by measuring the impedance change along a fracture site, and none of the references teach an apparatus capable of specifically detecting such impedance changes.